1. Field of the Invention
This invention lies in the field of telemetering electical signals from the bottom of a deep borehole to the surface. More particularly, it concerns the in situ measurement of temperature, pressure and/or other parameters, frequency modulating a high frequency carrier in accordance with the values of these parameters. This high frequency F.M. signal is generated and coupled to the bottom end of the power conductors that supply power to the drive motor. A coupling means picks off this high frequency F.M. signal at the surface and demodulates it to provide the original parameter values, which can then be displayed.
2. Description of the Prior Art
In the prior art there are a number of examples of instruments for making such parameter measurements at the bottom of a deep borehole, and transmitting them by means of cable to the surface where they can be utilized. Some of the transducers or sensors are of a well known design which produce a DC voltage or current which varies with the parameter. While these are simple instruments, they do offer considerable difficulty in their transmission because of the variable conditions along the conductor to the surface including variation of resistance of those conductors and insulation leakage.
Furthermore, in the particular conditions under which this instrumentation is to operate, there will be a submerged pump and drive motor, the power for which is carried down along side of the tubing that supports the pump and motor by means of a polyphase cable. The presence of the power conductors for the motor obviate the need for a separate conductor to carry the parameter signals. However, on the basis of the ohmmeter-type sensor construction, they still require careful processing of the data in order to avoid the variations in surface amplitude of signal due to varying losses along the cable.
Coupling a small direct current signal from the sensors to the motor power conductors in the power cable has heretofore required considerable coupling apparatus which is bulky and expensive.